Bandsawing is the preferred method for cutting long pieces of metal into predetermined lengths as a preparation for other operations such as welding, turning and milling.
When using bandsaw blades in maintenance and other smaller workshops it is common to cut a wide variety of work piece materials and sizes. It is not unusual that the operator of the saw machine is inexperienced and not properly trained in the use of the machine. Blades are seldom changed when cutting a new work piece, which is why the choice of bandsaw blade type and tooth pitch is not always done in the optimal way. The upkeep of the machine is often neglected, which is why cuts are frequently made without properly using the chip brush.
The consequence of these circumstances is that the saw blades are frequently prematurely destroyed. They are destroyed because the teeth are broken. The teeth break either because too coarse of a tooth pitch is used when cutting small work pieces such as thin walled tubes or profiles, or because too fine of a pitch is used when cutting large work pieces or when too high of a feed rate or too low of a band speed is used. When too coarse of a pitch is used, the teeth break because the teeth are overloaded. When too fine of a pitch is used, or when too high of a feed rate or too low of a band speed is used, or the chip brush is not properly used, the gullets are completely filled and excessive pressure is built up in the gullet, leading to tooth breakage.
Bandsaw blades for the above-described applications are normally designed with identical, repeating groups of teeth, wherein each group exhibits a variable pitch. This means that the distance between the teeth of each group varies in a predetermined way, each group repeating the same variation in tooth distance, so that harmonic vibrations are reduced when sawing. The ratio between the biggest tooth distance in the group and the smallest tooth distance is typically >1.4. Such a blade consists of groups of teeth, each group typically being of a length of 1.5″. The number of teeth in each group varies with the pitch of the blade but is typically odd in number. A blade with a 6/10 teeth per inch pitch typically has 11 teeth in each group. The meaning of the designation “ 6/10” is that the biggest tooth gullet in the group has a size corresponding to a regular 6 TPI blade, and the smallest gullet has a size corresponding to a 10 TPI blade. The clearance angle of the teeth is typically 35°, and the clearance side of the tooth, or the back of the tooth, is straight, leading to a small bottom radius of the gullet. The size of the bottom radius of the gullet is typically less than one-half of the depth of the gullet.
In deep gullets with small radii, the risk of swarf becoming locked in the gullet is increased. The swarf is often in the form of spiral shaped chips that get trapped between the sides of the deep, narrow gullets. The radius of such chips is typically about one-half of the depth of the gullets i.e., bigger than the radius of the gullets, thereby causing the chips to be locked in the bottom of the gullets.
Typically, most of the teeth are set, i.e., bent to the right or left. It is known to set the teeth within a group according to a predetermined setting pattern. The setting pattern of such a group normally follows the order: -0-R-L-R-L-R-L-R-L-R-L- wherein the first tooth in the group is unset (0), the next is set to the right (R), the following to the left (L), etc., there being only one unset tooth per group. The setting operation is done one group at a time. When setting-up the tooth-setting machine, it is of great importance that the correct tooth is left unset. If the wrong tooth is left unset, the setting hammers will hit the other teeth in the group in such a way that the teeth or the edges are deformed in an unwanted way. This leads to high amount of scrapped material in the production of such bandsaw blades.
The design of the gullets of traditional blades of this kind is such that all gullets in the group are proportional i.e. they have the same relationship between tooth distance, gullet depth and gullet radius. The chip holding capacity of each gullet in the group is proportional to the cross sectional area of the gullet. The chip holding capacity of the gullets in such a design is limited by the cross sectional area of the smallest gullet in the group. The other, bigger gullets in the group are unnecessarily big because all their dimensions are proportional to the tooth distance in relation to the smallest gullet.
A gullet that has a tooth distance that is 20% greater than the smallest gullet in the group will need 20% more chip capacity than the smallest gullet. By making all dimensions of the gullet proportional to the smallest gullet, the chip holding capacity will be 1.2×1.2=1.44, i.e., 44% more than the smallest gullet. This will result in gullet depths that are deeper than necessary, leading to reduced beam strength of the blade. Reduced beam strength will lead to out-of-square cutting at lower sawing loads compared to a blade with higher beam strength.